In a steering apparatus for a vehicle, if a driver rotates a steering wheel in a desired direction, a steering shaft connected to the steering wheel rotates, accordingly. The steering shaft delivers rotational force to a gear box including a rack-pinion gear through a universal joint. At this time, the gear box converts the rotational movement of the steering shaft to a linear movement through the rack-pinion gear so as to transfer it to a rack bar. The rack bar transfers force to a tie rod connected to a knuckle of a tire so as to allow change of the heading direction of a vehicle.
Particularly, a shaft positioned between the steering shaft and the gear-box has a structure where input and output shafts are connected with each other at a predetermined angle, and are not positioned on the same axis. This is because a typical type shaft assembling structure can not transfer power. Therefore, it is necessary to use a universal joint allowing the structure where a steering shaft can be tilted at a predetermined angle.
FIG. 1 is an exploded perspective view of a conventional universal joint, and FIG. 2 is a perspective view of a conventional slip bush.
As shown, the universal joint 100, which is connected with a steering shaft so as to deliver rotational movement of a steering wheel to a gear box, includes a tube 110 and a shaft 120. The shaft 120 has a structure which is inserted within the tube 110 by means of the slip bush 130.
The tube 110 has an inner circumferential surface which has been processed into a proper shape so as to allow an elastic part 140 with a C-shaped section and a solid part 150, which are formed along a circumferential surface of the slip bush, to be smoothly guided so that the tube is assembled with the slip bush 130 while making contact with an outer circumferential surface of the slip bush.
Moreover, one side of the shaft 120 has an outer circumferential surface which has been processed into a shape corresponding to the shapes of the elastic part 140 and the solid part 150, so as to be inserted into the slip bush 130. Therefore, the shaft 120 is assembled with the slip bush 130 while making contact with an inner circumferential surface of the slip bush.
As such, in the universal joint 100, at which the slip bush 130 is installed, the assembling structure of the tube 110 and the shaft 120 allows the length of the universal joint to be extended and contracted in an axial direction so as to absorb impact transferred from wheels and improve assembling effectiveness. The assembling structure absorbs kick-back load generated due to impact exerted to the wheels when a vehicle is driven on an uneven road surface, and simultaneously slides in an axial direction when the universal joint is assembled with a steering column and a gear box.
In general, the slip bush 130 is made from plastic material and has a cylindrical shape. Also, the slip bush 130 includes three elastic parts 140 and three solid parts 150, which are formed on circumferential surface of the slip bush 130 by turns while keeping a predetermined interval between each other.
The solid part 150 is shaped like a circular cylinder, in which a half part, which is shaped like a semi-circular cylinder, of the solid part is formed along an outer circumferential surface of the slip bush 130, and the other half part is symmetrically formed along an inner circumferential surface of the slip bush 130.
Also, the elastic part 140 is symmetrically formed along the outer and inner circumferential surfaces of the slip bush 130. The elastic part 140 has a cylindrical shape having a hollow interior, and a predetermined portion of the elastic part, which is formed along the outer circumferential surface of the slip bush 130, is cut out in a longitudinal direction so that the cross-section of the elastic part is shaped like “c”.
As such, in the structure of the conventional slip bush 130, when the shaft 120 slides to the interior of the tube 110 in a state where torsional torque or rotational torque is applied, the solid part 150 makes contact with the inner circumferential surface of the tube 110 so as to increase sliding frictional force through frictional force between the solid part 150 and the tube 110. Accordingly, the conventional slip bush 130 has such a structure, which can secure torsional rigidity.
However, in the conventional slip bush 130, the solid part 150 is formed integrally with the slip bush 130. Therefore, when the slip bush 120 slides in a state where rotational torque is applied to the shaft 120, a state where the whole outer circumferential surface of the solid part 150 makes contact with the tube 110 is maintained. As a result, the sliding frictional force increases, and a safety factor respective to torsional rigidity may be reduced.
In the conventional slip bush 130, the solid part 150 of the slip bush 130 receives a large part of load generated in a procedure where the rotational force is transferred between the shaft 120 and the tube 110. Therefore, when strong rotational force is applied to the slip bush during a short period of time, the solid part 150 of the slip bush 130 can be broken since the material of the slip bush 130 is equal to the material of the solid part 150. In this case, it is impossible to perform a steering operation, so that a serious problem regarding safety of a vehicle occurs.
If the conventional slip bush 130 is used, in a case where impact with torsion is exerted on the universal joint 100, sliding frictional force between the solid part 150 of the slip bush 130 and the inner circumferential surface of the tube 110 increases. Therefore, with the solid part 150 being worn away, clearance between the solid part 150 and the tube 110 increases, and in turn, generates unwanted noise.